1. Field of the Invention
The present invention relates to a reception circuit, a radio-controlled timepiece having the reception circuit, and a control method for the reception circuit.
2. Description of the Related Art
Radio-controlled timepieces that automatically correct the internally kept time to display the current time based on timecode information contained in a long-wave standard time signal are known from the literature. This type of radio-controlled timepiece has an internal reception circuit for receiving the long-wave standard time signal and outputting the time code. Japanese Unexamined Patent Appl. Pub. JP-A-2003-60520 (FIG. 2), for example, teaches a reception circuit that amplifies, detects, and demodulates a reception signal for the long-wave standard time signal received by the antenna.
The reception circuit (receiver) taught in JP-A-2003-60520 passes the long-wave standard time signal received by the antenna through an AGC amplifier and a crystal filter for filtering and tuning, then through a wave detection/rectification circuit and a wave-shaping circuit to input the timecode contained in the long-wave standard time signal to a control means. By switching a tuning switch so that a first coil bobbin is active, the inductance is adjusted to the 40-kHz reception frequency and long-wave standard time signals that are transmitted at 40 kHz can be received. If the tuning switch is set so that a second coil bobbin is active, the inductance is adjusted to the 60-kHz reception frequency and long-wave standard time signals that are transmitted at 60 kHz can be received. The receiver can thus receive long-wave standard time signals at different transmission frequencies.
The crystal filter in the reception circuit taught in JP-A-2003-60520 has two crystal oscillators, one operating at the 40-kHz oscillation frequency and the other at the 60-kHz oscillation frequency. By using the crystal oscillator tuned to the 40-kHz oscillation frequency to receive long-wave standard time signals transmitted at 40 kHz, and using the oscillator tuned to the 60-kHz oscillation frequency to receive long-wave standard time signals transmitted at 60 kHz, signals containing the desired frequency component can be extracted from the received standard time signal.
Japanese Unexamined Patent Appl. Pub. JP-A-2006-60849 teaches a reception circuit that uses a comparator as the wave detector for detecting the signal filtered by the crystal filter. This comparator compares the reception signal extracted from the received long-wave standard time signal with a specified reference voltage and outputs a binary signal.
The comparator used in the reception circuit (RF reception unit) taught in JP-A-2006-60849 outputs a HIGH level signal if the reception signal voltage is greater than the reference voltage, and outputs a LOW level signal if the reception signal voltage is lower than the reference voltage. The signal derived from the received long-wave standard time signal can thus be demodulated.
A problem with the reception circuit taught in JP-A-2003-60520 is that the filtered reception signal output from the crystal filter contains noise resulting from high frequency signal components, for example.
More specifically, a signal of the oscillation frequency of the crystal filter can be extracted from the reception signal by passing the reception signal through the crystal filter in the reception circuit taught in JP-A-2003-60520, but the extracted signal may also contain signal components at frequencies other than the oscillation frequency of the crystal filter. To handle such cases, a capacitor or other parallel capacitance could be disposed parallel to the crystal filter to cancel high frequency noise. However, if a crystal filter operating at the frequency of the received long-wave standard time signal and a parallel capacitance adjusted to the crystal filter are not provided in parallel, high frequency noise cannot be sufficiently cancelled and the extracted signal will contain noise.
A problem with the reception circuit taught in JP-A-2006-60849 is that the received long-wave standard time signal cannot be correctly demodulated if any deviation in the reference voltage input to the comparator is introduced when manufacturing the reception circuit.
More specifically, if noise caused by some other high frequency component is contained in the reception signal input to the comparator, the voltage may shift at the peak (high voltage level) and the bottom (low voltage level) of the reception signal. If the reference voltage is set to the median voltage between the peak voltage and the bottom voltage, noise will cause the reception signal to go above or below the reference voltage less frequently and the error rate of the binary signal will therefore drop. However, if the reference voltage is offset from the median between the top and bottom voltages of the reception signal, the effects of noise may cause a binary signal that is different from the signal that is actually carried by the standard time signal to be output. When this happens the standard time signal cannot be correctly demodulated.
This problem is exacerbated when the reception circuit can receive different long-wave standard time signals.
For example, if the peak voltage is 100%, the bottom voltage is set to 10% to receive the TCO signal of the Japanese standard time signal, but the bottom voltage is set to 25% to receive the TCO signal of the German standard time signal. As a result, if the reference voltage of the reception circuit is set according to the standard time signal used in Japan and the reception circuit is used to receive the German standard time signal, the voltage at the signal bottom cannot be correctly detected and the standard time signal cannot be correctly demodulated.